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Name: Russell
Status: N/A
Age: 20s

Location: N/A
Country: N/A
Date: 1999 


Question:
We know that blackbody radiation produces an intensity-frequency curve that is dependent on temperature. The idea of course is that you are looking into the cavity through a hole. Why isn't the outside of the blackbody also the same color as the inside (for a given temperature)? And what it we had a blackbody made of sodium. Wouldn't there be a peak in frequencies emitted that corresponds to the traditional sodium spectrum? Is then the blackbody curve "material-specific"? -Russell


Replies:
The blackbody curve itself is theoretical. Some materials come close to it. In practice, the radiation emitted from a cavity through a small hole ("cavity radiation") is very close to the theoretical blackbody curve for the same temperature. In the cavity, the radiation is essentially in equilibrium with the material - most of the radiation stays inside the cavity, being continually emitted and re-absorbed by the walls. Radiation emitted from the outer surface of a material will not necessarily be fully thermalized - some frequencies corresponding to certain transitions of the material, such as the sodium D line you mentioned, may be emitted preferentially. So, the blackbody curve is not material-specific, but the actual emission from an object will be. Cavity radiation will depend less on the material, and the smaller the hole, the closer it will correspond to the theoretical blackbody curve.

Richard E. Barrans Jr., Ph.D.
Assistant Director
PG Research Foundation, Darien, Illinois


A couple of comments:

A "Blackbody" is a theoretical construct which has a emissivity of exactly one at all wavelengths. This means that it absorbs all the energy that reaches it. It also means that its temperature is reflected in its emission spectrum (a Blackbody curve). The theoretical blackbody emission depends only on the temperature, not on the material. No material is a perfect blackbody.

Laboratory blackbody emitters are approximations of a true blackbody that consist of a heated chamber with a small hole to view the interior temperature. This configuration is used because it is much easier to control the temperature of the inside of an object than it is to control the temperature of its surface. If the interior of the chamber is at thermal equilibrium then absorption and emission of photons are at equilibrium for every wavelength. A small amount of energy escapes through the viewing hole and must be replenished by providing electricity to heat the chamber. The escaping energy (light) has the same spectral characteristics as the light inside the chamber -- which is a pretty good approximation of the blackbody radiation. The material of construction should not vaporize easily. Otherwise you will have cooler gas molecules around the exit from the chamber and they will superimpose characteristic absorption lines onto the blackbody spectrum.

Dr. Bradburn


The blackbody is built as you say, a cavity with a hole. The cavity is insulated, so as to let it hold a high temperature (200 C to 1000 C, perhaps). This can be a small hole, maybe just the size of a quarter. If the hole is too big, the cavity gets cooled by air, or whatever, and doesn't do exactly what its supposed to do.

Spectral lines such as you mention are in the visible range, sodium is yellow light. The black body is mainly used as a source of infrared radiatiion. It is not typically used for visible light, as there are other sources there. Plus the black body would have to be very hot to emit in much in the visible. If it were that hot I don't think you would get much absorption right at the spectral lines anyway, because the thing you describe would be made of solid sodium, not ionized. The black body curve itself is not material specfic, but if there is some gas between you and the black body then this would absorb. but people make these things out of common metals. Its just a big hot box. Dr. Ross



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